Query Optimization Patterns

Query Optimization Patterns — query optimization pattern guide

database query performance systematicas analysisand optimizationlower methodology.

execution plan analysis

PostgreSQL EXPLAIN

EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT o.*, c.name
FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE o.created_at > '2024-01-01'
ORDER BY o.total_amount DESC
LIMIT 10;

core metric

risk pattern detection

❌ Seq Scan on large_table (rows=10000000)
   → index addition necessary

❌ Sort Method: external merge (Disk: 256MB)
   → work_mem  or index sort

❌ Nested Loop (actual rows=1000000)
   → Hash Join or Merge Joinas transition

❌ estimated=100 actual=100000
   → ANALYZE executionto statistics renewal

index strategy

index typeper for

index principle

--   rule (Leftmost Prefix)
CREATE INDEX idx_orders ON orders(status, created_at, customer_id);

-- this index lower query:
✅ WHERE status = 'PAID'
✅ WHERE status = 'PAID' AND created_at > '2024-01-01'
✅ WHERE status = 'PAID' AND created_at > '2024-01-01' AND customer_id = 123
❌ WHERE created_at > '2024-01-01'  (status )
❌ WHERE customer_id = 123          (status, created_at )

-- column  decision criteria:
-- 1. etc. cases column  (optionalalso high )
-- 2. scope cases column 
-- 3. ORDER BY column 

index

-- table  this indexonlyas query completed
CREATE INDEX idx_covering ON orders(status, created_at) INCLUDE (total_amount);

SELECT total_amount FROM orders
WHERE status = 'PAID' AND created_at > '2024-01-01';
-- Index Only Scan  → heap  necessary

N+1 resolution

# N+1 pattern (!)
orders = Order.objects.filter(status="PAID")  # query 1
for order in orders:
    print(order.customer.name)  # query N (order countonly)
#  query: 1 + N

# Eager Loadingas resolution
orders = Order.objects.filter(status="PAID").select_related("customer")  # query 1 (JOIN)
# or
orders = Order.objects.filter(status="PAID").prefetch_related("items")  # query 2 (IN)

ORMper resolution

thisthis optimization

-- OFFSET (10000from → 10000  after )
SELECT * FROM orders ORDER BY id LIMIT 20 OFFSET 10000;

-- Keyset (immediate corresponding locationas)
SELECT * FROM orders WHERE id > 10000 ORDER BY id LIMIT 20;

query pattern

query optimization list

• EXPLAIN ANALYZE executionto execution plan confirmation
• Seq Scanthis ofalso-based confirmation ( data OK)
• estimated vs actual rows this confirmation
• necessary index
• N+1 query pattern without confirmation
• thisthisthis keyset confirmation
• necessary ORDER BY / DISTINCT removal
• transaction scope minimum confirmation